In some hoisting situations, it is difficult for a crane operator to determine if a crane is directly over the top of a load that is to be moved. In a side load situation, the crane is not directly over the point at which the hook/bottom block is attached to the load. Instead the bottom block may be offset horizontally some amount from its at-rest position. For example, suppose an operator intends to lift a load resting on the ground. If, after attaching the crane's hook to the load, the hook is displaced twelve inches to the side of its at-rest position, then when the operator hoists the load and the load leaves the ground, it may begin to swing. Loads can exceed 100,000 pounds, and can be very large as well. Swinging loads are hazardous because they can cause a number of potential issues, including cable damage creating a risk of cable breakage; damage to the load from impacting surrounding objects; damage to other loads or infrastructure; or injury or death to personnel on the ground hit or crushed by a swinging load.
If the hook is not correctly positioned over the load prior to hoisting, then the crane operator will often attempt to adjust the position of the crane so that the hook is vertically centered over the load, i.e., the hook is directly over the top of the center of gravity of the load. However, as has been mentioned, it is often difficult for an operator to determine if a hook is directly aligned above the load center. Even a small deviation from center can cause issues such as those described above.
In some situations, once a load has been moved, the crane is then moved to a different location. If an operator of the crane or ground personnel fail to ensure that the hook is disconnected from the load or the rigging, or fail to notice that the motion of the crane will take the hook into or through an area that has obstacles, a hook can snag. When a hook snags, motion of the hook can become unpredictable, and can lead to damage to the crane, cables, hook, and can cause serious injury or death, especially if the hook snags and drags something heavy or breakable.